Far infra-red pulsed beam devices operating in the terahertz frequency range have been described in various forms in the prior art. Devices have been used in time resolved infrared spectroscopy to characterize a variety of properties of solid state materials such as refractive index, photoconductivity, absorption, and dispersion.
Recent advances in terahertz beam generation can be attributed to optoelectronic interactions in semiconductor photoconductors. The advantage of this mechanism is that the radiation produced by this interaction can be radiated into free space. The beam can be steered using conventional optics to direct it onto samples for analysis, or imaging, and refracted or reflected to a photodetector operating on the same principle as the generator. A more thorough description of such a device is given by Smith et al, "Subpicosecond Photoconducting Dipole Antennas", IEEE Journal of Quantum Electronics, Vol. 24, No. 2,, pp. 255-260, February 1988. The device described by Smith et al uses a coplanar stripline terminating in a dipole antenna consisting of a small electrode gap formed over intrinsic silicon on sapphire. The electrode gap produces the high field photoconductive region. The pump beam was a mode locked dye laser pumped with an argon laser and operating with a pulse duration of 120 fs at 620 nm.
Another terahertz device is described by Van Exter et al in "Characterization of an Optoelectronic Terahertz Beam System", IEEE Transactions on Microwave Theory and Techniques, Vol. 38, No. 11, pp. 1684-1691, November 1990. Van Exter et al describe a device that uses a dipole antenna formed in the middle portion of a stripline. The substrate forming the photoconductive region in their device is also silicon on sapphire and they also use a colliding-pulse mode-locked dye laser as the pump source.
The terahertz generators/detectors in all of these devices is relatively simple in structure. Basically they comprise a semiconductor substrate with electrodes on the substrate and a small gap between the electrodes. With an appropriate DC bias applied to the electrodes, a field is established across the gap which produces a small high field photoconductor region near the surface of the semiconductor in the electrode gap. When this region is excited by fast pulses of light, rapid changes in conductivity occur. In the presence of the DC electric field these changes in conductivity result in ultrafast pulses of electric current through the dipole forming the gap, and equally ultrafast bursts of electromagnetic radiation are emitted from the gap region. Much of this radiation is emitted into the substrate, and since it has a photon energy well below the direct bandgap of the semiconductor, it can be collected by suitable lens arrangements on the obverse side of the semiconductor. The radiated beam can be collimated and focused using suitable mirrors, and can be detected by a device operating in a mode in reverse to that just described.
These THz devices were considered potentially useful in high speed pulse generators, high speed switching devices for communications and related applications, and in far infrared spectroscopy studies of different materials.
The potential usefulness of THz radiation for spectroscopy was later reported by Ralph et al in "Terahertz Beams: Generation and Spectroscopy", Mat. Res. Soc. Symp. Proc., Vol. 261, 1992, pp. 89-100, 1992. Much of this paper deals with the design and operation of the THz generator. Ralph et al analyzed the effect of the semiconductor properties on the electric field profile between the electrodes forming the emitter gap. They found that semi-insulating semiconductors produce enhanced field profiles. The desired semi-insulating property may be obtained through choice of a material with high trap density, or traps may be created in a normal semiconductor by ion beam damage. The device of Ralph et al used a semi-insulating substrate (GaAs) and the high field region was formed by relatively widely spaced (80 .mu.m) parallel electrode strips. The THz radiation produced by the apparatus described by Ralph et al was used to experimentally demonstrate THz time domain spectroscopy.
A more recent development in this technology was the proposal and demonstration of T-ray imaging by Hu and Nuss and reported in "Imaging with terahertz waves", Optics Letters, Vol. 20, No. 16, pp 1716-1718, August 1995. T-rays here refers to THz waves, and imaging with these waves is based on the fact that dielectric materials are quite transparent to this radiation, whereas water rich materials and metals are not. The maximum spatial resolution obtained in this work was limited by diffraction theory of far-field optics which in this case translates to roughly 150-400 .mu.m.
Following these published results interest increased in applications requiring effective imaging using THz beams. Pulsed THz devices have been used to characterize a variety of properties of solid state materials such as refractive index, photoconductivity, absorption, and dispersion.
Terahertz radiation imaging (T-ray imaging) shows promise in a variety of analytical applications such as chemical mapping, and a host of commercial applications such as safe package inspection, industrial process control, food inspection, biology and medicine. As radiation sensors, these devices are effective for analysis of solids, liquids or gases. Analysis of gases is particularly effective since gases have characteristically strong absorption lines in the THz frequency range. Accordingly, these devices can be used effectively for environmental studies and environmental monitoring.
Although these systems are characterized as T-ray imaging systems, they typically are applications that do not require high spatial resolution. It was recognized early that THz beams inherently have low spatial resolution, typically several hundred microns. Consequently their usefulness in many analytical applications, particularly in several sophisticated forms of time domain spectroscopy, requiring high spatial resolution, has thus far been limited.
In a recent advance in THz radiation imaging technology, near field imaging was pursued to overcome the resolution limitations of prior THz systems. Near field imaging allows spatial resolution below the wavelength of the radiation, i.e. sub-wavelength radiation. As described earlier, in a typical prior art THz generating device the emitter gap region is illuminated with a pump laser incident on the emitter gap surface, and far field radiation is emitted from the back side of the device substrate. This far field radiation can be converted to near field radiation using a small aperture converter device. This approach has been used successfully to produce scanning wavelengths well below the far field range. See e.g. S. Hunsche et al, "Near-Field THz Imaging", OSA Trends in Optics and Photonics Series (TOPS) Volume on Ultrafast Electronics and Optoelectronics (UEO TOPS 97').
In the near field system used by Hunsche et al, the near field converting device was a tapered metal tube, analogous to the tapered fibers used in near-field. scanning optical microscopes, and the tapered tube was used to aperture a conventional THz source beam to a spot smaller than the radiation wavelength. The tube was formed by electroplating a small conical aluminum tip with a Ni/Cr alloy, and polishing the tip to produce small circular apertures of the order of 50-100 .mu.m. This aperture diameter corresponds to .lambda./3 for a T-ray frequency of 1 THz. The THz beam was focused into the 2 mm opening of the tapered tube using off-axis paraboloid mirrors. While this technique has been successful, it requires somewhat complicated apparatus, and is lossy due to the multiple interactions of the THz beam with the beam collimating, steering, and near field converter surfaces.